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* Copyright 2017 gRPC authors.
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* See the License for the specific language governing permissions and
* limitations under the License.
// Package balancer defines APIs for load balancing in gRPC.
// All APIs in this package are experimental.
package balancer
import (
var (
// m is a map from name to balancer builder.
m = make(map[string]Builder)
// Register registers the balancer builder to the balancer map. b.Name
// (lowercased) will be used as the name registered with this builder. If the
// Builder implements ConfigParser, ParseConfig will be called when new service
// configs are received by the resolver, and the result will be provided to the
// Balancer in UpdateClientConnState.
// NOTE: this function must only be called during initialization time (i.e. in
// an init() function), and is not thread-safe. If multiple Balancers are
// registered with the same name, the one registered last will take effect.
func Register(b Builder) {
m[strings.ToLower(b.Name())] = b
// unregisterForTesting deletes the balancer with the given name from the
// balancer map.
// This function is not thread-safe.
func unregisterForTesting(name string) {
delete(m, name)
func init() {
internal.BalancerUnregister = unregisterForTesting
// Get returns the resolver builder registered with the given name.
// Note that the compare is done in a case-insensitive fashion.
// If no builder is register with the name, nil will be returned.
func Get(name string) Builder {
if b, ok := m[strings.ToLower(name)]; ok {
return b
return nil
// SubConn represents a gRPC sub connection.
// Each sub connection contains a list of addresses. gRPC will
// try to connect to them (in sequence), and stop trying the
// remainder once one connection is successful.
// The reconnect backoff will be applied on the list, not a single address.
// For example, try_on_all_addresses -> backoff -> try_on_all_addresses.
// All SubConns start in IDLE, and will not try to connect. To trigger
// the connecting, Balancers must call Connect.
// When the connection encounters an error, it will reconnect immediately.
// When the connection becomes IDLE, it will not reconnect unless Connect is
// called.
// This interface is to be implemented by gRPC. Users should not need a
// brand new implementation of this interface. For the situations like
// testing, the new implementation should embed this interface. This allows
// gRPC to add new methods to this interface.
type SubConn interface {
// UpdateAddresses updates the addresses used in this SubConn.
// gRPC checks if currently-connected address is still in the new list.
// If it's in the list, the connection will be kept.
// If it's not in the list, the connection will gracefully closed, and
// a new connection will be created.
// This will trigger a state transition for the SubConn.
// Connect starts the connecting for this SubConn.
// NewSubConnOptions contains options to create new SubConn.
type NewSubConnOptions struct {
// CredsBundle is the credentials bundle that will be used in the created
// SubConn. If it's nil, the original creds from grpc DialOptions will be
// used.
CredsBundle credentials.Bundle
// HealthCheckEnabled indicates whether health check service should be
// enabled on this SubConn
HealthCheckEnabled bool
// ClientConn represents a gRPC ClientConn.
// This interface is to be implemented by gRPC. Users should not need a
// brand new implementation of this interface. For the situations like
// testing, the new implementation should embed this interface. This allows
// gRPC to add new methods to this interface.
type ClientConn interface {
// NewSubConn is called by balancer to create a new SubConn.
// It doesn't block and wait for the connections to be established.
// Behaviors of the SubConn can be controlled by options.
NewSubConn([]resolver.Address, NewSubConnOptions) (SubConn, error)
// RemoveSubConn removes the SubConn from ClientConn.
// The SubConn will be shutdown.
// UpdateBalancerState is called by balancer to notify gRPC that some internal
// state in balancer has changed.
// gRPC will update the connectivity state of the ClientConn, and will call pick
// on the new picker to pick new SubConn.
UpdateBalancerState(s connectivity.State, p Picker)
// ResolveNow is called by balancer to notify gRPC to do a name resolving.
// Target returns the dial target for this ClientConn.
// Deprecated: Use the Target field in the BuildOptions instead.
Target() string
// BuildOptions contains additional information for Build.
type BuildOptions struct {
// DialCreds is the transport credential the Balancer implementation can
// use to dial to a remote load balancer server. The Balancer implementations
// can ignore this if it does not need to talk to another party securely.
DialCreds credentials.TransportCredentials
// CredsBundle is the credentials bundle that the Balancer can use.
CredsBundle credentials.Bundle
// Dialer is the custom dialer the Balancer implementation can use to dial
// to a remote load balancer server. The Balancer implementations
// can ignore this if it doesn't need to talk to remote balancer.
Dialer func(context.Context, string) (net.Conn, error)
// ChannelzParentID is the entity parent's channelz unique identification number.
ChannelzParentID int64
// Target contains the parsed address info of the dial target. It is the same resolver.Target as
// passed to the resolver.
// See the documentation for the resolver.Target type for details about what it contains.
Target resolver.Target
// Builder creates a balancer.
type Builder interface {
// Build creates a new balancer with the ClientConn.
Build(cc ClientConn, opts BuildOptions) Balancer
// Name returns the name of balancers built by this builder.
// It will be used to pick balancers (for example in service config).
Name() string
// ConfigParser parses load balancer configs.
type ConfigParser interface {
// ParseConfig parses the JSON load balancer config provided into an
// internal form or returns an error if the config is invalid. For future
// compatibility reasons, unknown fields in the config should be ignored.
ParseConfig(LoadBalancingConfigJSON json.RawMessage) (serviceconfig.LoadBalancingConfig, error)
// PickOptions contains addition information for the Pick operation.
type PickOptions struct {
// FullMethodName is the method name that NewClientStream() is called
// with. The canonical format is /service/Method.
FullMethodName string
// DoneInfo contains additional information for done.
type DoneInfo struct {
// Err is the rpc error the RPC finished with. It could be nil.
Err error
// Trailer contains the metadata from the RPC's trailer, if present.
Trailer metadata.MD
// BytesSent indicates if any bytes have been sent to the server.
BytesSent bool
// BytesReceived indicates if any byte has been received from the server.
BytesReceived bool
// ServerLoad is the load received from server. It's usually sent as part of
// trailing metadata.
// The only supported type now is *orca_v1.LoadReport.
ServerLoad interface{}
var (
// ErrNoSubConnAvailable indicates no SubConn is available for pick().
// gRPC will block the RPC until a new picker is available via UpdateBalancerState().
ErrNoSubConnAvailable = errors.New("no SubConn is available")
// ErrTransientFailure indicates all SubConns are in TransientFailure.
// WaitForReady RPCs will block, non-WaitForReady RPCs will fail.
ErrTransientFailure = errors.New("all SubConns are in TransientFailure")
// Picker is used by gRPC to pick a SubConn to send an RPC.
// Balancer is expected to generate a new picker from its snapshot every time its
// internal state has changed.
// The pickers used by gRPC can be updated by ClientConn.UpdateBalancerState().
type Picker interface {
// Pick returns the SubConn to be used to send the RPC.
// The returned SubConn must be one returned by NewSubConn().
// This functions is expected to return:
// - a SubConn that is known to be READY;
// - ErrNoSubConnAvailable if no SubConn is available, but progress is being
// made (for example, some SubConn is in CONNECTING mode);
// - other errors if no active connecting is happening (for example, all SubConn
// are in TRANSIENT_FAILURE mode).
// If a SubConn is returned:
// - If it is READY, gRPC will send the RPC on it;
// - If it is not ready, or becomes not ready after it's returned, gRPC will
// block until UpdateBalancerState() is called and will call pick on the
// new picker. The done function returned from Pick(), if not nil, will be
// called with nil error, no bytes sent and no bytes received.
// If the returned error is not nil:
// - If the error is ErrNoSubConnAvailable, gRPC will block until UpdateBalancerState()
// - If the error is ErrTransientFailure:
// - If the RPC is wait-for-ready, gRPC will block until UpdateBalancerState()
// is called to pick again;
// - Otherwise, RPC will fail with unavailable error.
// - Else (error is other non-nil error):
// - The RPC will fail with unavailable error.
// The returned done() function will be called once the rpc has finished,
// with the final status of that RPC. If the SubConn returned is not a
// valid SubConn type, done may not be called. done may be nil if balancer
// doesn't care about the RPC status.
Pick(ctx context.Context, opts PickOptions) (conn SubConn, done func(DoneInfo), err error)
// Balancer takes input from gRPC, manages SubConns, and collects and aggregates
// the connectivity states.
// It also generates and updates the Picker used by gRPC to pick SubConns for RPCs.
// HandleSubConnectionStateChange, HandleResolvedAddrs and Close are guaranteed
// to be called synchronously from the same goroutine.
// There's no guarantee on picker.Pick, it may be called anytime.
type Balancer interface {
// HandleSubConnStateChange is called by gRPC when the connectivity state
// of sc has changed.
// Balancer is expected to aggregate all the state of SubConn and report
// that back to gRPC.
// Balancer should also generate and update Pickers when its internal state has
// been changed by the new state.
// Deprecated: if V2Balancer is implemented by the Balancer,
// UpdateSubConnState will be called instead.
HandleSubConnStateChange(sc SubConn, state connectivity.State)
// HandleResolvedAddrs is called by gRPC to send updated resolved addresses to
// balancers.
// Balancer can create new SubConn or remove SubConn with the addresses.
// An empty address slice and a non-nil error will be passed if the resolver returns
// non-nil error to gRPC.
// Deprecated: if V2Balancer is implemented by the Balancer,
// UpdateClientConnState will be called instead.
HandleResolvedAddrs([]resolver.Address, error)
// Close closes the balancer. The balancer is not required to call
// ClientConn.RemoveSubConn for its existing SubConns.
// SubConnState describes the state of a SubConn.
type SubConnState struct {
ConnectivityState connectivity.State
// TODO: add last connection error
// ClientConnState describes the state of a ClientConn relevant to the
// balancer.
type ClientConnState struct {
ResolverState resolver.State
// The parsed load balancing configuration returned by the builder's
// ParseConfig method, if implemented.
BalancerConfig serviceconfig.LoadBalancingConfig
// ErrBadResolverState may be returned by UpdateClientConnState to indicate a
// problem with the provided name resolver data.
var ErrBadResolverState = errors.New("bad resolver state")
// V2Balancer is defined for documentation purposes. If a Balancer also
// implements V2Balancer, its UpdateClientConnState method will be called
// instead of HandleResolvedAddrs and its UpdateSubConnState will be called
// instead of HandleSubConnStateChange.
type V2Balancer interface {
// UpdateClientConnState is called by gRPC when the state of the ClientConn
// changes. If the error returned is ErrBadResolverState, the ClientConn
// will begin calling ResolveNow on the active name resolver with
// exponential backoff until a subsequent call to UpdateClientConnState
// returns a nil error. Any other errors are currently ignored.
UpdateClientConnState(ClientConnState) error
// ResolverError is called by gRPC when the name resolver reports an error.
// UpdateSubConnState is called by gRPC when the state of a SubConn
// changes.
UpdateSubConnState(SubConn, SubConnState)
// Close closes the balancer. The balancer is not required to call
// ClientConn.RemoveSubConn for its existing SubConns.
// ConnectivityStateEvaluator takes the connectivity states of multiple SubConns
// and returns one aggregated connectivity state.
// It's not thread safe.
type ConnectivityStateEvaluator struct {
numReady uint64 // Number of addrConns in ready state.
numConnecting uint64 // Number of addrConns in connecting state.
numTransientFailure uint64 // Number of addrConns in transientFailure.
// RecordTransition records state change happening in subConn and based on that
// it evaluates what aggregated state should be.
// - If at least one SubConn in Ready, the aggregated state is Ready;
// - Else if at least one SubConn in Connecting, the aggregated state is Connecting;
// - Else the aggregated state is TransientFailure.
// Idle and Shutdown are not considered.
func (cse *ConnectivityStateEvaluator) RecordTransition(oldState, newState connectivity.State) connectivity.State {
// Update counters.
for idx, state := range []connectivity.State{oldState, newState} {
updateVal := 2*uint64(idx) - 1 // -1 for oldState and +1 for new.
switch state {
case connectivity.Ready:
cse.numReady += updateVal
case connectivity.Connecting:
cse.numConnecting += updateVal
case connectivity.TransientFailure:
cse.numTransientFailure += updateVal
// Evaluate.
if cse.numReady > 0 {
return connectivity.Ready
if cse.numConnecting > 0 {
return connectivity.Connecting
return connectivity.TransientFailure
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